The present invention relates to a cleaning apparatus and a cleaning method of cleaning a substrate to be cleaned such as a semiconductor substrate (semiconductor wafer), a liquid crystal glass substrate, or a magnetic disk and, more particularly, to a cleaning apparatus and a cleaning method which achieve a remarkable cleaning effect in cleaning steps in the manufacture of silicon single-crystal wafers or cleaning steps during the course of fabricating semiconductor devices by using these wafers.
Semiconductor wafers, liquid crystal glass substrates, and magnetic disks must be cleaned whenever they are processed because various films and patterns are formed on them.
The degree of integration of semiconductor devices using semiconductor wafers as base materials continues to increase year by year. Accordingly, it is becoming more and more important to keep a high cleanliness in an environment in which semiconductor devices are fabricated and to clean wafers as the base materials.
One principal purpose of wafer cleaning is to remove particles and contamination, such as metal impurities, organic substances, surface films (natural oxide films and surface adsorbed substances) sticking to the wafer surface. A second purpose is to make the planarization of the wafer surface more perfect to eliminate troubles in the fabrication of semiconductor devices, thereby increasing the fabrication yield and improving the reliability as devices.
As conventional wafer cleaning methods, RCA cleaning developed by Mr. Kern et al. of RCA Corp. in the 1960s is best known to those skilled in the art. A representative cleaning sequence based on this method has two stages. In the first stage, particles and organic substances are removed by an SC-1 (RCA Standard Clean-1) cleaning solution based on ammonia/hydrogen peroxide/water (NH.sub.4 OH/H.sub.2 O.sub.2 /H.sub.2 O). In the second stage, metal contamination is removed by an SC-2 (RCA standard Clean-2) cleaning solution based on hydrogen chloride/hydrogen peroxide/water (HCl/H.sub.2 O.sub.2 /H.sub.2 O). DHF (Dilute HF) cleaning for removing surface films by using hydrogen fluoride/water (HF/H.sub.2 O) is also combined with the above basic combination in some cases.
The SC-1 cleaning solution used in the RCA cleaning method removes particles and organic substances from the wafer surface by using an etching action of ammonia contained in the cleaning solution.
Unfortunately, a silicon single-crystal ingot pulled by the CZ method contains crystal defects called as-grown defects formed during the growth of the ingot. Therefore, when the resultant wafer surface is etched, portions having these defects are etched faster than portions having no defects. Consequently, the etching of the defective portions selectively proceeds to form fine pits in the wafer surface. When a laser beam is irradiated on the surface of the wafer having these pits to measure particles on the wafer surface by using a particle counter for measuring the scattered light (bright points), the scattered light from each pit is also detected. This makes the measurement of a true particle count impossible. This pit is called a COP (Crystal Originated Particle).
The COP can deteriorate the electrical breakdown voltage characteristic of a gate oxide film of a semiconductor device. Therefore, as the packing density of semiconductor devices continues to increase, a countermeasure against the COP which has not been given serious consideration has also become an important problem to be solved.
The SC-1 cleaning solution has a high removing effect on a metal such as Cu which easily forms a complex with ammonia. However, the detergency of the SC-1 cleaning solution to metal impurities is generally inferior to those of cleaning solutions using acids.
On the other hand, the SC-2 cleaning solution has a remarkable cleaning effect on metal impurities, although its detergency to particles and organic substances is weak. However, a silicon oxide film is formed on the wafer surface due to hydrogen peroxide contained in the chemical solution. Therefore, the effect of this cleaning solution is believed to weaken if the concentration of metal impurities is high.
The cleaning method using the chemical solutions as described above dissolves metals such as exposed aluminum used as interconnections in a cleaning step during the course of fabricating semiconductor devices. Also, during this cleaning even interconnections buried in interlayers may corrode through fine gaps or pinholes. This imposes limitations on the use of the cleaning method.
As described above, the conventional cleaning methods represented by the RCA method have the problems stemming from the chemical solutions such as ammonia and acids used in the cleaning steps. Additionally, three or four types of cleaning solutions having different cleaning purposes and effects are used in combination. This necessarily prolongs the cleaning steps and requires multiple stages of apparatuses and multiple chemical solutions. Multiple steps resulting from the chemical solution cleaning eventually increase the installation cost, the labor cost, and the cost of materials such as chemical solutions and pure water. Also, the cost of countermeasures against environmental pollution such as waste liquors reaches a considerable amount.
To solve these problems, a cleaning method by which the use of chemical solutions is minimized or, if possible, no chemical solutions are used, is described in Jpn. Pat. Appln. KOKAI Publication No. 6-260480.
That is, in the invention described in the above publication, a cathode bar and an anode bar are disposed in two chambers, isolated by a porous film, of an electrolytic bath. Pure water containing a small amount of a substance which increases the electrolytic efficiency is supplied into the two isolated chambers to produce OH.sup.- ionized water on the cathode side and H.sup.+ ionized water on the anode side. These two ionized waters are introduced to two processing baths provided as separate chambers to clean objects to be cleaned.
In this cleaning method, the processing on the OH.sup.- ionized water side is deemed equivalent to the SC-1 cleaning in the RCA cleaning method described above. Jpn. Pat. Appln. KOKAI Publication No. 6-260480 describes that OH.sup.- ions exclusively stabilize the surface of aluminum activated by the H.sup.+ ion processing or removes colloidal silica remaining after a polishing or planarization process. However, this publication does not describe the removal of general particles as the principal object of the conventional SC-1 cleaning nor a more positive effect as a countermeasure against the COP which is a shortcoming of the method.
On the other hand, the processing on the H.sup.+ ionized water side is equivalent to the SC-2 cleaning in the RCA cleaning method because the purpose of the processing is to exclusively remove metal contamination from a silicon wafer. However, the above publication describes that the applied voltage in this processing must be a high DC voltage of 10.sup.3 to 10.sup.4 V/cm, so the processing is extremely dangerous. Also, the OH.sup.- ionized water and the H.sup.+ ionized water produced in the two isolated chambers consist of unstable ions and return to neutral solutions with a passage of time. Therefore, the concentrations of these ionized waters lower before the waters reach the two processing baths provided as separate chambers. Consequently, the effects of these ionized waters weaken or their concentrations become difficult to control.